Method and apparatus for observing and inspecting defects

ABSTRACT

A defect inspecting apparatus is disclosed that can detect finer defects with high resolution optical images of those defects, and which makes the difference in contrast greater between fine line patterns of a semiconductor device. The defect inspecting apparatus includes a sample mounting device for mounting a sample; lighting and detecting apparatus for illuminating a patterned sample mounted on a mount and detecting the optical image of the reflected light obtained therefrom. Also included is a display for displaying the optical image detected by this lighting and detecting apparatus; an optical parameter setting device for setting and displaying optical parameters for the lighting and detecting apparatus on the display; and optical parameter adjusting apparatus for adjusting optical parameters set for the lighting and detecting apparatus according to the optical parameters set by the optical parameter setting apparatus; a storage device for storing comparative image data; and a defect detecting device for detecting defects from patterns formed on the sample by comparing the optical image detected by the optical image detecting apparatus with the comparative image data stored in the storage.

BACKGROUND OF THE INVENTION

This invention relates to a high resolution optical system used forinspecting and observing fine pattern defects, foreign matter, etc,which appear, for example, in manufacturing processes of semiconductordevices and flat panel displays. The invention also relates to a defectinspecting apparatus that uses such a high resolution optical system.

A conventional technique, which has provided a method and an apparatusfor photographing the structures of fine lines in width using an opticalmicroscope, is disclosed in Japanese Patent Laid-Open No. 7-128595. Thistechnique is characterized by the use of light which is linearlypolarized by a polarizer positioned at about 45° to the linear dimensionof a sample. The optical delay axis of a ¼ wavelength plate placedbetween the polarizer and the sample is angled optimally (25° typically)to the main linear shape of the sample. This ¼ wavelength plate convertsthe linearly polarized light to elliptically polarized light, which isthen applied to the sample. This elliptically polarized light, whenreflected from the sample, has a phase difference. The reflected lightpasses through the ¼ wavelength plate again, then passes through apolarizer provided in a detecting light path. The light passing throughthe polarizer forms an image of the sample on a photoelectric conversionelement. In such conventional apparatus, therefore, the phase differencecaused by the sample is estimated beforehand, enabling the light set aselliptic polarized light to be converted to circularly polarized lightafter reflection from the sample.

In the method and apparatus for imaging structures of fine line widthusing an optical microscope as described, a polarizer is disposed in alighting light path, and a linearly polarized light is passed throughthe polarizer. Then this linearly polarized light is converted to anelliptic polarized light through the ¼ wavelength plate before it isapplied to the sample. In such an optical system, therefore, both0-order diffracted light reflected from the sample and higher-orderdiffracted light become circularly polarized light, and the ratiobetween the amplitudes of the 0-order diffracted light effective forforming optical images and higher-order diffracted light is the same asthat of the random polarized light (the amplitude of the 0-orderdiffracted light is larger than that of the higher-order diffractedlight). Consequently, the 0-order diffracted light and the higher-orderdiffracted light interfere with each other, thereby degrading theresolution of the optical image of the sample, particularly because theamplitude of the higher-order diffracted light is small, and this causesthe low frequency component to be increased.

If an image sensor is used for detecting images, the light intensity isadjusted so as to prevent saturation. For example, because a cyclicalfine pattern increases the diffraction angle, the contrast betweenpatterns is small, making detected images dark. If a pattern image isdetected and this detected image is processed for defect inspection, thesmall difference in contrast between patterns, and the dark image causeslower detection sensitivity.

SUMMARY OF THE INVENTION

The present invention provides an optical system that can control thepolarization of both 0-order diffracted light used to form opticalimages and higher-order diffracted light, thereby detecting objectpatterns for inspection at a high resolution, enabling detection offiner defects.

To achieve this, the invention provides a method for observing a samplewith patterns formed thereon. The method includes applying a polarizedlight to the sample through an objective lens, detecting the polarizedlight applied to and reflected from the surface of the sample throughthe objective lens, thereby calculating a deviation of the polarizedlight from the focal point on the surface of the sample in the axialdirection thereof, then adjusting the height of the sample to theobjective lens according to the calculated deviation from the focalpoint, and detecting the polarized light reflected from the surface ofthe height-adjusted sample through the objective lens, as well as aphase difference plate and an analyzer.

The present invention also provides another method for observing asample with patterns formed thereon using an optical system. The methodincludes the steps of applying a polarized light to the sample from theoptical system set on a first polarizing condition, detecting the lightapplied to and reflected from the surface of the sample through a phasedifference plate and an analyzer to thereby obtain a first image,displaying the first image on a monitor screen, setting the opticalsystem on a second polarizing condition according to the displayed firstimage, applying the polarized light to the sample while the opticalsystem is set on the second polarizing condition, and detecting thelight applied to and reflected from the surface of the sample throughthe phase difference plate and the analyzer, thereby obtaining thesecond image.

The present invention also provides an apparatus for observing a samplewith patterns formed thereon. The apparatus comprises a light source forapplying a polarized light to the sample through an objective lens, afocal point detector for detecting the light applied to and reflectedfrom the surface of the sample through the objective lens, andcalculating a deviation of the polarized light from the focal point onthe surface of the sample in the axial direction thereof, a heightadjustment device for adjusting the height of the sample to theobjective lens according to the deviation from the focal point,calculated by the focal point detecting means, and a polarized lightdetector for detecting the light reflected from the surface of thesample through the objective lens, as well as a phase difference plateand an analyzer when the polarized light is applied from the lightingmeans to the sample whose height is adjusted by the height adjustingmeans.

In another embodiment, the present invention provides a method forinspecting defects of a sample with patterns formed thereon. The methodincludes the steps of applying a polarized light to the sample throughan objective lens, detecting the polarized light applied to andreflected from the surface of the sample through the objective lens, aswell as a phase difference plate and an analyzer, thereby obtaining animage of the surface of the sample, then comparing the obtained imagewith a corresponding image stored beforehand, thereby detecting defectsof the sample.

In yet another embodiment, the present invention also provides a methodfor inspecting defects of a sample with patterns formed thereon whichmethod includes the steps of applying polarized light to the sample froman optical system set on a first polarizing condition, detecting thelight applied from the optical system and reflected from the surface ofthe sample through a phase difference plate and an analyzer to therebyobtain a first image, displaying the first image on a monitor screen,setting the optical system on a second polarizing condition according tothe first image displayed on the monitor screen, applying polarizedlight to the sample while the optical system is set on the secondpolarizing condition, detecting the light applied to and reflected fromthe surface of the sample through the phase difference plate and theanalyzer to thereby obtain a second image, then comparing the secondimage with a corresponding image stored earlier so as to detect defectsof the sample.

Further, the present invention also provides apparatus for inspectingdefects of a sample with patterns formed thereon, the apparatuscomprising a light for applying a polarized light to the sample throughan objective lens, a polarized light image detector for detecting thelight reflected from the surface of the sample through the objectivelens, as well as a phase difference plate and an analyzer when thepolarized light is applied to the sample to thereby obtain an image ofthe sample, and a defect detector for comparing the image obtained bythe polarized light image detector with a corresponding image storedbeforehand so as to detect defects of the sample.

Further, the present invention also provides a high resolution opticalsystem comprising an optical system for applying a polarized light to asample, an optical part for polarization for passing a higher-orderdiffracted light, which is polarized and rotated by the sample, moreefficiently than the 0-order diffracted light, and an optical system fordetection, used for forming an image of the sample on a photoelectricconversion element with the light passing or reflected from the opticalpart for polarization. (For example, the high resolution optical systemapplies a polarized light to the sample with oscillations orthogonal toa line pattern of the sample and the optical system is provided with apolarizer disposed so as to assume the vibrating direction of 450 to thepattern, as a transmission axis.)

This polarized light can originate from a light passing or reflectedfrom a polarized light beam splitter. If such a light is used, a ½wavelength plate and a ¼ wavelength plate are disposed between thepolarized light beam splitter and the sample, so that the ½ wavelengthplate is rotated according to the orientation of the pattern, etc.,thereby rotating the polarizing direction, and the ¼ wavelength plate isrotated, thereby adjusting the ellipticity of polarization.

Further, the present invention allows the combination of the 0-orderdiffracted light used to form optical images and the direction of thehigher-order diffracted light to be varied in many ways. The combinationis important for making the high resolution optical system practical.The high resolution optical system is provided with functions forcollecting images detected respectively by changing the polarizationstatus and carrying out a preliminary defect inspection so as to selecta polarizing condition for improving the defect inspect sensitivity. Theoptimal value for the polarizing condition can thus be found correctlyand quickly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a schematic front cross-sectional view of an embodiment ofa high resolution optical system of the present invention;

FIG. 1(b) illustrates light on an object lens and diffracted lightreflected from a sample;

FIG. 1(c) shows polarization of the illumination source and polarizationof diffracted light reflected from the sample;

FIG. 2 illustrates detection results of optical images of patternsdetected by the conventional down-lighting and by an embodiment of thepresent invention;

FIG. 3 is a schematic front cross-sectional view of a defect inspectionapparatus in a first embodiment of the present invention;

FIG. 4 is a schematic front cross-sectional view of a defect inspectionapparatus in a second embodiment of the present invention;

FIG. 5(a) is a schematic front section view of a defect inspectionapparatus in a third embodiment of the present invention;

FIG. 5(b) shows the wavelength range for detecting a focal point;

FIG. 5(c) shows a spectrum reflection factor of the second dichroicmirror;

FIG. 6 illustrates a recipe display and an optical parameter settingdisplay;

FIG. 7(a) is a top view of a wafer to be inspected;

FIG. 7(b) is an expanded top view of part of the wafer shown in FIG.7(a);

FIG. 7(c) is a cross-sectional view of the wafer, which is cut at line125;

FIG. 8 illustrates a sample flowchart for setting optical parameters;

FIG. 9 illustrates diffracted light images and pattern images;

FIG. 10(a) is a schematic cross-sectional view of an embodiment of alighting/detecting optical system of the present invention, which uses apolarized light;

FIG. 10(b) is an image of levels of gray detected in normal bright-fielddetection;

FIG. 10(c) shows the distribution of the brightness at the A-A line inthe image of levels of gray;

FIG. 10(d) shows the distribution of the brightness at the A-A line ofthe levels of gray image shown in FIG. 10(b), which is detected byadjusting the phase difference in an embodiment of the presentinvention; and

FIG. 11 is a chart showing the relationship between ellipticity anddetected light ratio or contrast.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

An embodiment of the high resolution optical system of the presentinvention will be described with reference to FIG. 1(a). A light 180emitted from a light source 8 is condensed by a lens 9 and passedthrough a polarizer 14. The light 180 thus becomes linearly polarizedlight and passes through a half-mirror 15. After passing throughhalf-mirror 15, the light 180 strikes a sample 1 through an objectivelens 20. Light 180 is reflected form the sample I and becomes diffractedlight 189. Part of the diffracted light 189 enters the NA (NumericalAperture) of the objective lens 20, then reflected from the half-mirror15 to an image sensor 70. Detected light 189, when oscillating in thedirection of the electric field vector (hereafter, the polarizeddirection of the light) on the polarizing plane corresponding to thetransmission axis of the polarizer 22, where it forms an image of thesample I on the image sensor 70 through image-forming lens 30.

The polarizer 14 determines the direction in which the polarized lightvibrates with respect to the direction 1 a of the main pattern formed onthe sample 1. (The main pattern is often the result of an upper layer ofconductive material patterned into fine metal lines.) Therefore, thepolarizer 14 is positioned so as to polarize light in an orthogonaldirection to the direction 1 a of the pattern to detect. This plane ofthe polarized light is in the direction at the injection eye 19 of theobjective lens 20. The polarizer 22 passes part of the detected light189, to be transmitted in a specific vibrating plane to the patternorientation 1. For example, polarizer 22 is set to an angle of 45°.

Next, we describe the light source, as well as the polarization of the0-order diffracted light and higher-order diffracted light reflectedfrom the sample 1 with reference to FIG. 1(b). We also describe thepolarization of the light applied to the sample 1, as well as thepolarization of the 0-order diffracted light and the higher-orderdiffracted light reflected from the sample 1, all with reference to FIG.1(c).

In FIG. 1(b) it is assumed that the light polarized at one point of theillumination light 180 on the injection lens is polarized as shown by300. This polarized light 300 matches, or is orthogonal to, thedirection 301 for sample 1. Thus, the polarization 310′ of the reflectedlight (0-order diffracted light) and the polarization 311 of thehigher-order diffracted light are the same as the polarization of theillumination source. The linear polarization is converted to ellipticalpolarization, because a phase difference is created when the light isreflected and diffracted from sample 1.

If the polarization direction 300′ of the light 180 does not match, noris orthogonal to, the direction for illumination of sample 1, thepolarization direction 310′ of the 0-order diffracted light matches withthe polarization direction 300′ of the illumination source. Thepolarization direction 320 of the higher-order diffracted light differsfrom the polarization direction 300′ of the illumination source due torotation of polarization. (The actual polarization direction isdistributed and the polarization direction mentioned here is thedirection of the largest amplitude.) Consequently, as shown in FIG.1(d), the polarization direction 310′ of the 0-order diffracted lightdiffers from the polarization direction 320 of the higher-orderdiffracted light. It is thus possible to increase the amplitude of thehigher-order diffracted light on the image surface by matching thetransmission axis of the polarizer 22 with the polarization direction320 of the higher-order diffracted light. Consequently, the 0-orderdiffracted light and the higher-order diffracted light become almostequal in amplitude and the resolution of the optical image formed due tothe interference between the 0-order diffracted light and thehigher-order diffracted light is improved.

FIG. 2 shows diffracted light images and optical images of a pattern oflines and spaces when defects are detected using a conventional downlighting method and using a polarized light method of the presentinvention, respectively. The transmission axes of the polarizers 14 and22 used for obtaining images using the polarized light method of thepresent invention are the same as those shown in FIG. 1.

Numeral value 1 is set for σ in both the conventional down lightingmethod and the polarized lighting method of the present invention. Thediffracted light image obtained by the conventional down lighting methoddistributes the 0-order diffracted light. The approximately first-orderdiffracted light is darker, and has less amplitude than the 0-orderdiffracted light.

In contrast, if the polarized lighting method of the present inventionis used, the diffracted light image obtained will be greater than thefirst-order diffracted light in amplitude, and detected as a brighterimage. The approximately first-order diffraction image also includes the0-order diffracted light. More precisely, the first-order diffractionimage is the sum of the 0-order diffracted light corresponding to thepositive first-order diffracted light and the negative first-orderdiffracted light distribution. This is also true for the positive1-order diffracted light image. And, the reduced portion of the 0-orderdiffracted light is not detected in the diffracted light image at theinjection point 19 of the objective lens 20, because the correspondinghigher-order diffracted light is outside the NA of the objective lens20. (The diffraction angle of this first-order diffracted light isdetermined by both wavelength and pattern width.)

Accordingly, the light at the reduced portion of the 0-order light is alow frequency component not used to form the image of the sample 1.Thus, the image resolution can be improved due to such reduction. Withthe conventional down-lighting method it is difficult to separate linesand spaces from each other, because the contrast of the lines and spaces(found from the peak (a) and the bottom (b) of the detected contrastdifference signal) is low (C=0.028). The method of the presentinvention, however, can separate lines and spaces from each otherclearly, because in one embodiment the contrast is improved to 0.178.According to the present invention, it will be understood that theresolution is improved significantly compared to the conventionalmethod.

FIG. 3 illustrates a first embodiment of a defect inspecting apparatusthat uses the optical system of the present invention. A sample 1, forexample, a wafer, is held by a vacuum chuck 2. Chuck 2 is mounted on aθ-stage 3 so as to be above a Z-stage 4, a Y-stage 5, and an X-stage 6,respectively. An optical system 111 disposed above the sample 1 is usedto detect the optical image of the sample 1, enabling inspection of theexternal appearance of each pattern formed on sample 1. The opticalsystem 111 mainly comprises a lighting system, a detection opticalsystem for photographing the image of the sample 1, and a focal pointdetection optical system 45.

A light source 8 in the lighting optical system is an incoherent lightsource, for example, a xenon lamp. The light emitted from light source 8is transmitted through the aperture of a diaphragm 11 and through a lens9. The light, which passes through a wavelength selecting filter 12 andanother lens, reaches a visual field diaphragm 13. This wavelengthselecting filter 12 limits the wavelength range, thereby detecting ahigh resolution image of the sample 1 by taking the spectral reflectionfactor of the sample 1 into consideration. For example, an interferencefilter is employed. The light, which passes through the visual fielddiaphragm 13, passes through a polarizer 14 to become a linearlypolarized light. This polarized light then passes through a half-mirror15 and enters an objective lens 20 to illuminate sample 1.

After illuminating the sample 1, the light is reflected, scattered, anddiffracted by the sample, and part of the light enters the objectivelens 20 again and reaches the half-mirror 15. The light is thenreflected from the half-mirror 15 and enters the polarizer 22. Thepolarizer 22 is positioned around the reflected light axis so as totransmit the higher-order diffracted light more than the 0-orderdiffracted light. Passing through the polarizer 22, the light passesthrough the image forming lens 30 and the zoom lens 50, then forms animage on the light-receiving surface of the image sensor 70. The imagesensor 70 is, for example, a linear sensor, a TDI image sensor, a TVcamera, or other imaging device.

At this time, the focal point detection light must be provided to thefocal point detecting optical system 45 for automatic focusing of theobject image. The light dividing means 25, for example, can be adichroic mirror. The focal point detection light forms an optical imagehaving the height information of the sample 1 on the sensor 41 throughthe image forming lens 40. The output signal from the sensor, whichdetects this optical image, is supplied to a focal point detectionsignal processing circuit 90. Circuit 90 detects the deviation betweenthe height of the sample 1 and the focal point of the objective lens 20,thus transmitting the detected deviation to the CPU 75. Using thisdeviation value, the CPU 75 instructs the stage controller 80 to drivethe Z-stage 4 so the stage controller 80 transmits a predetermined pulseto the Z-stage 4. The automatic focusing function is thus stated.

The image signal obtained by detecting the optical image of the sample 1at a photoelectric conversion element 70 is provided to an imageprocessing circuit 71 so as to store the image and determine defects.The X-stage 6 and the Y-stage 5 are used for moving the sample in twodimensions in the X-Y directions. The 0-stage 3 is used for rotationalalignment of the pattern formed by the sample 1.

Although the optical system of the down-lighting method has beendescribed in this embodiment, the resolution R of this optical system isgenerally found from an expression of R=λ/(2NA). As shown in FIG. 2,however, an optical system that uses linearly polarized light can obtaina higher resolution than R.

Next, second embodiment of the defect inspecting apparatus of thepresent invention will be described with reference to FIG. 4. The lightemitted from a light source 8 passes through an aperture diaphragm 11via a lens 9 and enters a polarized light beam splitter 15 via awavelength selection filter 12 and a visual field diaphragm 13. Thelight, which passes a λ/2 plate (½ wavelength plate) 16 and a λ/4 plate(¼ wavelength plate), illuminates the sample 1 through an objective lens20. The λ/2 and λ/4 plates provide a phase difference to the P-polarizedlight passing through the polarized light beam splitter 15. By rotatingthe λ/2 plate 16 around the light axis, the polarizing direction of thepolarized light is set at a predetermined angle (e.g., 90° shown in FIG.1). The λ/4 plate 17 is a phase difference plate for transforming thelinearly polarized light to elliptically polarized light. The detectedlight, which illuminates sample 1 through the objective lens 20, passesthrough the λ/2 and λ/4 plates 16 and 17 again and is reflected by beamsplitter 15, and it is led into the detected light path.

If no λ/4 plate 17 is used in the optical configuration shown in FIG. 4,the light reflected positively (0-order diffracted light) by the sample1 passes through the polarized light beam splitter 15. Therefore, onlythe higher-order diffracted light is provided to the detecting lightpath. If a polarized light beam splitter 15 is used and provided withthe same functions as those of the polarizers 14 and 22 in theconfiguration shown in FIG. 3, a λ/4 plate 17 provides 0-orderdiffracted light having the same amplitude as the higher-orderdiffracted light to the detecting light path.

If 45° is taken between the light axis of the crystal of the λ/4 plate17 and the polarizing direction of the illumination, the light passingthrough the λ/4 plate 17 becomes circularly polarized light. In thiscase, the light intensity supplied to the detecting light path isincreased. This is because the light intensity of the 0-order diffractedlight, which is a low frequency component, is increased. As a result,the contrast of the image is lowered more than the linearly polarizedlight. This is why the optimized angle between the light axis of thecrystal of the λ/4 plate 17 and the polarization direction of theillumination source is changed by, for example, the phase difference ofthe sample 1. Accordingly, the λ/4 plate 17 should be allowed to rotateto enable the phase difference of the sample 1 to be changed. If theplate 17 is so composed, part of the light reflected and diffracted bythe sample 1 passes through the λ/2 and λ/4 plates 16 and 17 again andis reflected by the polarized light beam splitter 15, thereby forming anoptical image of the sample 1 on the image sensor 70.

The third embodiment of the defect inspecting apparatus of the presentinvention is described with reference to FIG. 5. In this embodiment anillumination source of less than 200 to 250 nm in wavelength isemployed. Because resolution is improved with a shorter wavelength, theuse of an ultra-violet light or a DUV (Deep Ultra Violet) light iseffective to further improve resolution. The optical system shown inFIG. 5(a) allows a light emitted from an ultra-violet beam source 8 topass to a beam splitter 15 through optical system 112. The polarizedlight passing through this beam splitter 15 passes the λ/2 and λ/4plates 16 and 17, so that the light has a phase difference. The lightthen strikes sample 1 from above through the objective leans 20. Thedown-lighting consists of an ultra-violet light for the visual field anda focal point light for detecting the height of the sample 1.

The focal point light is effective if it is visible light, which is notabsorbed by a flattening film. If light is completely absorbed by aflattening film, the height of the sample 1 cannot be detected. Thewavelength for this focal point detecting light is determined by thespectrum reflection factor of the dichroic mirror 25. FIG. 5(b) shows anexample in which light of not less than 650 nm in wavelength is used assuch a focal point detecting light.

The light path for detecting bright-field images using an ultra-violetlight is a light path reflected from the second dichroic mirror 26. Thelight path detects images on the surface of a sensor 70 a through animage forming lens 30 a. This ultra-violet beam wave range is determinedby the spectrum reflection factor of the second dichroic mirror as shownin FIG. 5(c).

If the ultra-violet beam is applied to the sample 1, the sample 1generates a fluorescent light depending on the material. Thisfluorescent light can be used to detect defect images. In this case, thefluorescent light makes it possible to detect even the defects thatcannot be detected in bright-field images. Such an optical system can beput into practical use if it is composed so as to form an intermediateimage through an image forming lens from a light passing two dichroicmirrors and project this image expanded by a zoom lens 50 on the imagesensor 70. An objective lens for ultra-violet lights can correct theaberration with the ultra-violet beam, but the focal point detectinglight should be a light with less aberration from the ultra-violet bean.

FIG. 6 illustrates how to set optical parameters for inspecting defectsof a pattern using the defect inspecting apparatus of the presentinvention, for example, as used in a semiconductor manufacturingprocess. At first, an inspection recipe is displayed on the screen. TheID of a wafer to be inspected according to this displayed recipe isregistered. After that, the type of wafer is entered. This makes itpossible to identify the type of each defect, as well as the process inwhich the defect is detected. Furthermore, the inspection area isspecified with, for example, coordinates of the object wafer. Althoughthis defect inspection method detects each defect by obtaining the imageof a pattern formed on the object wafer, there are also other inspectionmethods, including a method for detecting defects by detectingdifferences by comparison between images of adjacent chips, and a methodfor detecting differences as a result of comparison between design dataand an object image. One of these methods is selected for “InspectionMethod.” Then, the threshold of a difference image, is entered so as toposition image and decide the defect detection sensitivity for “ImageProcessing Parameters.” The defect detection sensitivity is also set soas to change the “Optical Parameters.”

Changing the “Optical Parameters” is next described. The menu “OpticalParameters” is selected on the recipe screen to display the “OpticalParameters” setting screen. On this “Optical Parameters” setting screen,necessary parameters are set by selecting from 1, PolarizationCharacteristics; 2, Wavelength; 3, Lighting σ; and 4, Space Filter, etc.The choice (1, Polarization Characteristics) decides the polarization ofboth the lighting optical system and the detecting optical system with,for example, a number. If the number for specifying polarizationcharacteristics for higher defect detection sensitivity is alreadyknown, the number is entered. However, since such polarizationcharacteristics for higher defect detection sensitivity are not definedyet in the initial inspection of a wafer, polarization characteristicseffective for improving the defect detection sensitivity are selectedaccording to the object pattern shape. The relationship between such apattern shape and polarization characteristics is found, for example,from the relationship shown in FIG. 1(a).

The choice “2, Wavelength” selects a light wave range for a highercontrast for defects. For example, a high wavelength range is selectedfor a higher reflection factor of the object pattern to inspect.Otherwise, a wavelength range should be selected for a larger differenceof brightness between the pattern and the background (no-patternregion). In the case of a wafer treated with chemical mechanicalpolishing (CMP), if the film thickness is uneven, the brightness of thedetected image becomes uneven due to the interference of the insulatingfilm. Since the unevenness of this film thickness is not a defect, thedifference of brightness appears as a noise in the defect inspection. Toreduce such a brightness difference, increasing the lighting wavelengthrange is effective. However, because an increase of wavelength rangediffers between the design thickness of the insulating film and theerror, the choice of a lighting wavelength range from the insulatingfilm thickness is effective. This is why the thickness of the objectinsulating film is entered as a condition for selecting the lightingwavelength range.

The “Lighting σ” selects an aperture diameter of the aperture diaphragm11 of the lighting system. If a hole-like pattern is formed on thesample 1, the lighting a value should be set smaller than that of theline-like pattern so as to improve the defect detection sensitivity.

A space filter is an optical filter for reducing the amplitudetransmission factor of the 0-order diffracted light or giving a phrasedifference to the 0-order diffracted light at a Fourier transformationsurface (the injection point of an objective lens or at a positioncommon to this injection point (the position of the zoom lens 50)).Disposition of such a space filter according to the shape, density,etc., of the object pattern enables the resolution of the image to beimproved. If the optical parameters are set as described, the image issensed more effectively for detecting defects. Whether or not the setconditions are proper, however, depends on the shape of the objectpattern and the structure of the object wafer.

As shown in FIG. 7(a), patterns are disposed regularly on a wafer inunits of an exposure field of the object. FIG. 7(b) shows an expandedview of the 1′ portion of the wafer 1. The wafer 1 has patterns 1 a and1 a′ to be inspected, as well as a pattern 1 c, etc., formed on theprevious process. FIG. 7(c) shows a cross-sectional view of the wafer 1,which is cut at the line 125. It is assumed that the patterns 1 a and 1a′ to be inspected are formed on an insulating film 1 b, and the pattern1 c, not to be inspected, is formed in the insulating film 1 b. In thiscase, the patterns 1 a and 1 b are to be inspected mainly in thevertical direction. Thus, if optical parameters are set so as toincrease the contrast of the object pattern in this direction, thedefect detection sensitivity can be improved. However, since the optimalvalue of each of those optical parameters depends on the shape andstructure of the object pattern, time is required to determine such anoptical value.

FIG. 8 is an optical parameter setting flowchart for setting opticalparameters quickly and effectively. At first, a wafer is loaded in theinspecting apparatus, then the inspection area is moved into the visualfield of the optical system. After that, a necessary parameter range forobtaining a preliminary image is entered for each of the opticalparameters (1, Polarization Characteristics; 2, Wavelength; 3, Lightinga; and 4, Space Filter). Next, each image is obtained using the setconditions and displayed so as to easily check diffracted images andpattern images. In addition, the total sum of secondary differentialvalues indicating pattern contrast and pattern sharpness, etc., aredisplayed. Consequently, optical parameters, as well as optical imageresolution information can be listed to enable optimal values of opticalparameters to be decided easily.

The final optical parameters are decided from a viewpoint of whether ornot the object image is effective to detect defects. Consequently, theobject optical parameter range is narrowed from the contrast and the sumof secondary differential values mentioned above, and finally theoptical parameters are decided by changing those parameters while apreliminary inspection is carried out. As a result of such a preliminaryinspection, for example, optical parameters are decided according to theinconsistent values (average, maximum, and deviation) of a differenceimage in a normal portion and the result of comparative inspection(detected defect count, defect signal level, S/N, which is a ratiobetween the maximum inconsistency value N in a normal portion and anunmatching count in a defect portion S, and other factors). According tosuch, the optical parameters setting process, at least one of thenecessary optical parameters is set, thereby enabling other opticalparameters to be set effectively for a high defect detectionsensitivity.

FIG. 9 is an example of displaying diffracted light images/patternimages shown in the list of optical conditions and optical images in theoptical parameters setting process flow shown in FIG. 8. As an exampleof displaying a series of diffracted light images/pattern images, thepolarization status is assumed as a parameter. At first, thepolarization direction of a polarized light may be defined for thedirections of the XY stages and the orientation flat of the wafer. Forexample, 90° and 45° of the polarization direction of the illuminatinglight and the detecting light mean angels between the main direction ofan object pattern to inspect and the polarization direction of the mainlight beam.

This display makes it possible to estimate a ratio between the amplitudeof a higher-order diffracted light and the amplitude of the 0-orderdiffracted light from a diffracted light image. In addition, if thereare many patterns to inspect in various directions, it is possible todetect each of those pattern directions, the contrast, etc., frompattern images. In addition, if the light intensity distribution of animage is displayed with a polygonal line graph, etc., it is easier torecognize the brightness level, etc. In addition, it will be understoodfrom the contrast and the sum of secondary differential values that theresolution of images is improved at around 90° and 68° of thepolarization directions of the illumination light. Consequently, if thenext preliminary inspection is carried out on this condition, opticalconditions for enabling a high sensitivity inspection can be setquickly.

FIG. 10(a) shows detecting bright-field and dark-field images bypolarization, and detecting neutral images of the bright-field anddark-field images. Light 180 is randomly-polarized light. If the light180 is provided to the polarized light beam splitter 15, only the lighthaving the appropriate polarized light component passes through thesplitter 15. This polarized light is then passed through the λ/2 and λ/4plates 16 and 17, thereby changing both the rotation of the polarizationdirection and the ellipticity of the polarized light. The illumination,when reflected from the sample 1, is given a phase difference accordingto the phase jump and the sample pattern, thereby changing thepolarization status. If a light is diffracted at an edge of a pattern,etc., the polarization status is also changed according to the directionof the diffracted light. These reflections are caught by the objectivelens 20 and passed to the polarized light beam splitter 15 through theλ/2 and λ/4 plates 16 and 17. The polarized light component is reflectedfrom the splitter 15 so as to form a bright-field image on the sensor70.

If a detected image is a levels-of-gray image as shown in FIG. 10(b) ina normal bright-field detection, the brightness distribution at the lineA-A will be as shown in FIG. 10(c), and the image becomes dark in thedense pattern portion on the left side of the image. Although thecontrast of the dense pattern portion can be increased by increasing thelight intensity, the brightness in the flat portion on the right side ofthe image is further increased, saturating the CCD sensor 70 and causingblooming. This is why the light intensity cannot be increased, and thedense pattern cannot be detected without proper contrast. However, ifthe phase difference is adjusted using the λ/2 and λ/4 plates 16 and 17,it is possible to reduce the light intensity on each flat portion, whichis reflected from the polarized light beam splitter as shown in FIG.10(d) and increase the reflection of the diffracted light from the densepattern portion. The brightness both at a dense pattern and at a flatportion can thus be detected on the same level.

It is therefore possible to detect high contrast images regardless ofthe pattern density, etc., and detect pattern defects, etc., easily evenin a dense pattern portion of the wafer. To achieve this, if linearlypolarized light is used to illuminate the sample 1, the polarizationdirection of the polarized light and the orientation of the pattern areset at right angles. The polarization direction of the polarized lightcan be rotated by rotating the λ/2 plate.

For a wafer on which a CMP treatment is used, the brightness can becomeuneven due to the uneven thickness of the film, mainly in areas of lowerpattern density. Because this brightness unevenness is not a fataldefect, the noise appearing during defect detection can be reduced byreducing the detected light intensity at a flat portion. Consequently,if the reflected light intensity from a flat portion is set lower thanthat from a high pattern density portion, it is possible to increase thedefect signal of the high pattern density portion whose fatality iscomparatively high, and reduce the noise caused by a reflected lightfrom the flat portion whose fatality is comparatively low. As describedabove, defects can be detected even with CMP wafers at a highsensitivity.

In the optical system as illustrated in FIGS. 3, 4, 5 and 10, linearlypolarized light reaches a ¼ wavelength plate 17 and is converted intoelliptically polarized light. The elliptically polarized light strikesthe sample 1 through the objective lens 20. Light reflected or refractedfrom the sample is collected by the object lens 20 and its phase isshifted by passing through the ¼ wavelength plate 17 again.

By rotating the ¼ wavelength plate 17, the ellipticity of theelliptically polarized light which illuminates the sample 1 is changed.The relation between an ellipticity and image contrast of the sample 1detected by the image sensor 70 is illustrated in FIG. 11. Theellipticity is for distinguishing the polarizing direction, clockwise orcounterclockwise. The contrast of the image is determined from an imageof periodically repeating line patterns formed on the surface of thesample 1. When the light intensity is constant, the amount of lightdetected by the image sensor 70 decreases with the reduction of theabsolute number of the ellipticity. A light intensity ratio is definedas a relative amount of light compared to the circularly polarized light(its ellipticity is 1 or −1).

In FIG. 11, when the ellipticity is −0.18, the contrast is maximized.Therefore, to detect a defect of periodically repeated line patterns, itis desirable to set the ellipticity of the illumination light atapproximately −0.18. On the other hand, the light intensity ratiodecreases to 0.1 at that ellipticity. So, when detecting a defect bysetting the ellipticity of the illumination at approximately −0.18, theintensity of light emitted from the light source 8 needs to be about tentimes more than that of when the ellipticity is 1 or −1, a circularlypolarized illumination light.

In some cases, ellipticity of the illumination light source to havemaximum inspection sensitivity is not the same to that which has amaximum contrast. It is desirable to determine the ellipticity of lightas follows. At first, detecting a defect several times by setting theellipticities of the illuminating light between −0.4 and −0.1, where thecontrast of image is relatively high. Then, from the result of thedetection, determining the ellipticity of light to obtain a maximumdefect detecting sensitivity.

The technique for realizing a high resolution can also apply to aprojection type exposure unit to be employed for patterningsemiconductors, etc. More specifically, a polarization element isdisposed between reticule and wafer so that a polarized lightilluminates the reticule, which is an original sheet, and the amplitudeof the 0-order diffracted light passing through the reticule becomesequal to the amplitude of a higher-order diffracted light, forming ahigh contrast reticule image on a wafer coated with a resist. The marginof the resist development is thus increased and thereby improvingproductivity.

As described above, according to the present invention, it is possibleto provide an optical system, which can control the angle between thepolarization direction of the electrical field vector and theorientation of the object pattern on the polarizing plane ofillumination according to the object pattern formed on the sample,thereby providing high resolution images of defects so as to detect morefine defects. The present invention also makes it possible to set apolarization direction of the electrical field vector on the polarizingplane, light wavelength, etc., more efficiently, thereby providing ahigher resolution optical system and a defect inspecting apparatus witha high sensitivity using such a high resolution optical system.

The preceding has been a description of the preferred embodiment of theinvention. It will be appreciated that deviations and modifications canbe made without departing from the scope of the invention, which isdefined by the appended claims.

1. A method for observing a sample with patterns formed thereon,including the steps of: applying a polarized light to the sample throughan objective lens; detecting the light applied to and reflected from thesurface of the sample through the objective lens, thereby calculating adeviation of the polarized light from a focal point on the sample in theaxial direction of the light; adjusting the height of the sample to theobjective lens according to the calculated deviation from the focalpoint; and detecting the polarized light applied to and reflected fromthe surface of the height adjusted sample through the objective lens, aphase difference plate and an analyzer.
 2. A method for observing asample in accordance with claim 1 wherein the phase difference platechanges part of the intensity of the light reflected from the surface ofthe sample.
 3. A method for observing a sample in accordance with claim1 wherein the phase difference plate reduces the amplitude of only the0-order diffracted light reflected from a pattern formed on the surfaceof the sample more than the amplitudes of higher-order diffracted lightsbeing included in the light reflected form the surface of the sample. 4.A method for observing a sample in accordance with claim 1 wherein themethod further includes a step for displaying the image of the sampleformed with the detected light, on a monitor screen.
 5. A method forobserving a sample with patterns formed thereon, including the stepsfor: applying a polarized light to the sample from an optical system seton first polarizing condition; detecting the light applied to andreflected from the surface of the sample through a phase differenceplate and an analyzer, thereby obtaining a first image; displaying thefirst image on a monitor screen; setting the optical system on a secondpolarizing condition according to the first image displayed on themonitor screen; applying a polarized light to the sample from theoptical system set on the second polarizing condition; and detecting thelight applied to and reflected from the surface of the sample throughthe phase difference plate and the analyzer, to thereby obtain a secondimage.
 6. A method for observing a sample in accordance with claim 5wherein the first and second polarizing conditions include polarizingcharacteristics, polarized light wavelength, the aperture of an aperturediaphragm of a lighting system, and the shape of a space filter.
 7. Amethod for observing a sample in accordance with claim 5 wherein thephase difference plate is adjusted when the optical system is set in thesecond polarizing status, thereby changing the intensity of the lightapplied to and reflected from the surface of the sample.
 8. A method forobserving a sample in accordance with claim 5 wherein the phasedifference plat is adjusted when the optical system is set in the secondpolarizing condition, thereby reducing the amplitude of the 0-orderdiffracted light from a pattern formed on the surface of the sample morethan the amplitude of the higher-order diffracted light, both of the0-order and higher-order diffracted light being included in the lightreflected from the surface of the sample.
 9. A method for observing asample in accordance with claim 5 wherein the method further includes astep for displaying the image of the sample formed with the detectedlight on a monitor screen.
 10. A method for observing a sample inaccordance with claim 9 wherein the method further includes a step fordisplaying information comprising polarizing characteristics, polarizedlight wavelength, the aperture of the aperture diaphragm of the lightingsystem, and the shape of the space filter.
 11. An apparatus forobserving a sample with patterns formed thereon including: lightingmeans for applying a polarized light to the sample through an objectivelens; focal point detecting means for detecting the light applied fromthe lighting means and reflected from the sample through the objectivelens, thereby calculating a deviation of the polarized light from thefocal point on the surface of the sample in the axial direction thereof;height adjusting means for adjusting the height of the sample to theobjective lens according to the deviation from the focal point,calculated by the focal point detecting means; and polarized lightdetecting means for detecting the light reflected from the surface ofthe sample through the objective lens, a phrase difference plate and ananalyzer when the light polarized by the lighting means is applied tothe sample whose height is adjusted by the height adjusted means.
 12. Anapparatus for observing a sample in accordance with claim 11 wherein thephase difference plate has a ½ wavelength plate and a ¼ wavelength plateand is used to change the intensity of the light reflected from thesurface of the sample.
 13. An apparatus for observing a sample inaccordance with claim 11 wherein the phase difference plate has a ½wavelength plate and a ¼ wavelength plate and is used to reduce theamplitude of the 0-order diffracted light from a pattern formed on thesurface of the sample more than the amplitude of the higher-orderdiffracted light, both of the 0-order and higher-order diffracted lightbeing included in the light reflected from the surface of the sample.14. An apparatus for observing a sample in accordance with claim 11wherein the apparatus further includes display means for displaying theimage of the sample formed with the detected light.
 15. A method forinspecting defects of a sample with patterns formed thereon, the methodincluding steps for: applying polarized light to the sample through anobjective lens; detecting the polarized light applied to and reflectedfrom the surface of the sample through the objective lens, a phasedifference plate and an analyzer, thereby obtaining an image of thesample; and comparing the obtained image with a corresponding imagepreviously stored, thereby detecting the defects of the sample.
 16. Amethod for inspecting defects of a sample in accordance with claim 15wherein the phase difference plate changes the intensity of thepolarized light applied to and reflected from the surface of the samplepartially.
 17. A method for inspecting defects of a sample in accordancewith claim 15 wherein the phase difference plate reduces the amplitudeof the 0-order diffracted light from a pattern formed on the surface ofthe sample more than the amplitude of the higher-order diffracted light,both of the 0-order and higher-order diffracted lights being included inthe light applied to and reflected from the surface of the sample.
 18. Amethod for inspecting defects of a sample in accordance with claim 15wherein the method further includes a step for displaying the image ofthe sample formed with the detected light on a monitor screen.
 19. Amethod for inspecting defects of a sample with patterns formed thereon,including the steps for: applying a polarized light to the sample froman optical system set on first polarizing condition; detecting the lightapplied to and reflected from the sample, to thereby obtain a firstimage; displaying the first image on a monitor screen; setting theoptical system in a second polarizing condition according to the firstimage displayed on the monitor screen; applying the polarized light tothe sample while the optical system is est in the second polarizingcondition; detecting the light applied to and reflected from the sample,thereby obtaining a second image; and comparing the obtained secondimage with a corresponding image previously stored, thereby detectingdefects of the sample.
 20. A method for inspecting defects of a samplein accordance with claim 19 wherein the first and second polarizingconditions are polarizing characteristics, the wavelength of thepolarized light, aperture of the aperture diaphragm of the lightingsystem, and the shape of the space filter.
 21. A method for inspectingdefects of a sample in accordance with claim 19 wherein the methodchanges the intensity of the light applied to and reflected from thesample partially when the optical system is set in the second polarizingstatus.
 22. A method for inspecting defects of a sample in accordancewith claim 19, wherein the method reduces the amplitude of the 0-orderdiffracted light from a pattern formed on the surface of the sample morethan the amplitude of the higher-order diffracted light when the opticalsystem is set in the second polarizing status, both of the 0-order andhigher-order diffracted light being included in the light applied to andreflected from the surface of the sample.
 23. A method for inspectingdefects of a sample in accordance with claim 19 wherein the methodfurther includes a step for displaying the image of the sample formedwith the detected light, on a monitor screen.
 24. A method forinspecting defects of a sample in accordance with claim 19 wherein themethod further includes a step for displaying such information aspolarizing characteristics, the wavelength of the polarized light, theaperture of the aperture diaphragm, and the shape of the space filter onthe monitor screen.
 25. An apparatus for inspecting defects of a samplewith patterns formed thereon including: lighting means for applying apolarized light to the sample through an objective lens; polarized lightimage detecting means for detecting the light reflected from the surfaceof the sample through an objective lens, a phase difference plate and ananalyzer when the polarized light is applied to the sample from thelighting means, thereby obtaining the image of the sample; and defectdetecting means for comparing the image obtained by the image detectingmeans with a corresponding image stored beforehand, thereby detectingdefects of the sample.
 26. An apparatus for inspecting defects of asample in accordance with claim 25 wherein the phase difference platehas a ½ wavelength plate and a ¼ wavelength plate and is sued to changethe intensity of the light applied to and reflected from the surface ofthe sample.
 27. An apparatus for inspecting defects of a sample inaccordance with claim 25 wherein the phase difference plate has a ½wavelength plate and a ¼ wavelength plate and is used to reduce theamplitude of the 0-order diffracted light from a pattern formed on thesurface of the sample more than the amplitude of the higher-orderdiffracted light, both of the 0-order and higher-order diffracted lightsbeing included in the light applied to and reflected from the surface ofthe sample.
 28. An apparatus for inspecting defects of a sample inaccordance with claim 25 wherein the apparatus is further provided withdisplay means for displaying the image of the sample formed with thedetected light on a monitor screen.